Tanning process for forming a photographic relief image

ABSTRACT

A PROCESS IS DISCLOSED FOR FORMING A PHOTOGRAPHIC RELIEF IMAGE FROM A COPY MEDIUM WHICH HAS A BASE COATED WITH A GELATIN BINDER WHICH CONTAINS A PHOTOCONDUCTOR. THE PHOTOCONDUCTOR IS OF THE TYPE WHICH BECOMES REVERSIBLY ACTIVATED UPON BEING STRUCK BY LIGHT OR OTHER ACTIVATING RADIATION AND IS CAPABLE OF CAUSING THE REDUCTION OF REDUCIBLE IONS IN ITS ACTIVATED STATE. THE COPY MEDIUM IS FIRST EXPOSED TO AN IMAGE PATTERN OF ACTIVATING RADIATION SUCH AS VISIBLE LIGHT TO REVERSIBLY ACTIVATE THE PHOTOCONDUCTOR AT THE IMAGE-STRUCK AREA. THE EXPOSED AREAS ARE SUBSEQUENTLY CONTACTED WITH IMAGE-FORMING MATERIALS WHICH CONTAIN REDUCIBLE IONS SUCH AS SILVER IONS TO FORM A GERM IMAGE. AFTER THE GERM IMAGE HAS BEEN FORMED THE HARDENABLE BINDER IS CONTACTED WITH A TANNING DEVELOPER WHICH HARDENS THE BINDER AT THE IMAG-STRUCK AREAS. REMAINING PORTIONS OF THE UNHARDENED BINDER ARE WASHED AWAY WHICH LEAVES A RELIEF IMAGE FORMED FROM HARDENED GELATIN AT THE IMAGE-STRUCK AREA. THE PHOTOGRAPHIC RELIEF IMAGE FORMED BY THE PROCESS OF THIS INVENTION IS USEFUL AS A PHOTGRAPHIC TRANSPARENCY, DIAZO MASTER, DYE TRANSFER MATRIX, LITHOGRAPHIC PLATE, ETC.

United States Patent Corporation, Lexington, Mass. N0 Drawing. Filed May 19, 1969, Ser. No. 825,994

Int. Cl. G03c 5/00 US. CI. 96-35 17 Claims ABSTRACT OF THE DISCLOSURE A process is disclosed for forming a photographic relief image from a copy medium which has a base coated with a gelatin binder which contains a photoconductor. The photoconductor is of the type which becomes reversibly activated upon being struck by light or other activating radiation and is capable of causing the reduction of reducible ions in its activated state. The copy medium is first exposed to an image pattern of activating radiation such as visible light to reversibly activate the photoconductor at the image-struck area. The exposed areas are subsequently contacted with image-forming materials which contain reducible ions such as silver ions to form a germ image. After the germ image has been formed the hardenable binder is contacted with a tanning developer which hardens the binder at the image-struck areas. Remaining portions of the unhardened binder are washed away which leaves a relief image formed from hardened gelatin at the image-struck area.

The photographic relief image formed by the process of this invention is useful as a photographic transparency, diazo master, dye transfer matrix, lithographic plate, etc.

BACKGROUND OF THE INVENTION (1) Field of the invention This invention relates to a process for forming photographic relief images.

(2) Description of the prior art The formation of latent and visible images using certain photoconductors such as titanium dioxide which can be reversibly activated by light is described in Gold, US. Pat. 3,380,823. In the Gold process, the photoconductor is reversibly activated by exposure to activating radiation and subsequently physically developed by contacting the exposed photoconductor with a solution of readily reducible ions and a developer. One common image-forming material used with photosensitive photoconductive materials of this type is a solution of noble metal salts, particularly a silver salt such as silver nitrate.

The use of tanning developers to form relief images is well-known in the silver halide art. See, for example, US. Pats. 3,345,170 and 3,402,045 and Canadian Pat. 775,505. Also, an indirect method for using tanning developers with photosensitive materials as described in Gold has been described in our co-pending application Sprague, U.S. Ser. No. 713,212 and now abandoned. In the indirect method for tanning, the photoconductor material is exposted, developed and oxidized by a bleaching solution prior to tanning.

Heretofore, tanning physical development of photoconductive materials as described in Gold has not been used. This is probably because tanning agents usually require highly alkaline conditions, while the physical developers are usually used in acidic solutions.

SUMMARY OF THE INVENTION According to the present invention, it has been unexpectedly found that a copy medium having a base with a ice photoconductor dispersed in a hardenable binder thereon can be used to directly produce a photographic relief image by the steps of exposing, forming a germ image, tanning and removing the untanned portions. The photoconductor is of the type that becomes reversibly activated upon exposure to activating radiation and capable of reducing reducible ions when it is reversibly activated. One example of such a photoconductor is titanium dioxide.

By the process of this invention, the copy medium is first exposed to an image pattern of activating radiation which serves to reversibly activate the photoconductor at the image-struck area. The image-struck area is subsequently contacted with image-forming materials which contain readily reducible ions such as silver ions to form a germ image. After the germ image is formed, the imagestruck area is contacted with tanning developer which hardens the gelatin binder at the image-struck area. The untanned portions of the gelatin emulsion are removed by washing or otherwise to leave a photogarphic relief image.

There are many advantages to the process of this invention for forming photographic relief images. For example, the process of this invention provides a direct process for forming photographic relief images from copy media containing photosensitive photoconductors without going through the intermediate steps of bleaching the silver image, re-exposing and redeveloping the bleached silver image as has been done heretofore. Since fewer steps are required, this process is simpler and more economical.

Use of a copy medium which has a reversibly activatable photoconductor as the photosensitive element means that the copy medium does not have to be manufactured or handled in the dark, as is absolutely necessary with silver halide copy media. It also means that processing (13f the medium toobtain relief images can be very rapi DESCRIPTION OF THE PREFERRED EMBODIMENTS The copy media used to form photographic relief images by the process of this invention have a base with a hardenable binder containing a photosensitive photoconductor thereon. The base can be any supporting substrate such as paper, metal, glass, plastic, film or the like. If the relief image is to be used as a transparency, the base is preferably translucent or transparent. Film with excellent dimensional stability is particularly preferred as base materials such as that polyester film sold under 21c trademark-Mylar by E. I. du Pont de Nemours &

Photocond-uctors useful with this invention are those which can be reversibly activated upon exposure to activating radiation and which when reversibly activated are capable of causing the reduction of reducible ions. The photoconductors preferred in this invention are metal containing photoconductors. A preferred group of such photosensitive photoconductor materials. are the inorganic materials such as compounds of a metal and a nonmetallic element of Group VI-A of the Periodic Table such as: oxides including but not limited to zinc oxide, titanium dioxide (TiOz), zirconium dioxide, germanium dioxide, indium trioxide; and metal selenides such as cadmium selenide. Metal oxides are especially preferred photoconductors. Titanium dioxide is a preferred metal oxide because of its unexpectedly good results, and titanium dioxide having an average particle size less than about 250 millimicrons and which has been treated in an oxidizing atmosphere at a temperature exceeding about 200 C. for a period of at least one-half hour and up to 30 hours or more is especially preferred. This preferred titanium dioxide is further described in US. Patent applications Ser.

3 Nos. 432,887 and 463,037, and now abandoned, the teachings of which are herein incorporated by reference.

Also useful in this invention as photoconductors are certain fluorescent materials. Such materials include, for example, compounds such as silver activated zinc sulfide, and zinc activated zinc oxide.

While the exact mechanism by which the photoconductors of this invention work is not known, it is believed that exposure of the photoconductors of this invention to activating radiation causes an electron or electrons to be transferred from the valence band of the photoconductor to the conductance band of the same or at least to some similar excited state whereby the electron is loosely held, thereby changing the photoconductor from an inactive form to an active form. The activation of these photoconductors is reversible and inactivation can be accomplished by methods specified infra. If the active form of the photoconductor is in the presence of an electron accepting compound, a transfer of electrons will take place between the photoconductor and the electron accepting compound, thereby reducing the electron accepting compound. Therefore a simple test which may be used to determine whether or not materials are photoconductors which can be reversibly activated is to mix the photoconductor in question with an aqueous solution of silver nitrate. Little, if any, reaction should take place in the absence of light. The mixture is then subjected to light such as ultraviolet light. At the same time, a control sample of an aqueous solution of silver nitrate alone is subjected to the light. If the mixture darkens faster than the silver nitrate alone, that material is a photoconductor which can be reversibly activated by activating radiation.

It is evident that the gap between the valence and the conducting band of a compound determines the energy needed to make electron transitions. The more energy needed, the higher the frequency to which the photoconductor 'will respond. It is known to the art that it is possible to reduce the band-gap for these compounds by adding a foreign compound as an activator which either by virtue of its atomic dimensions or by possessing a particular electronic forbidden zone structure or through the presence of traps as donor levels in the intermediate zone between the valence and the conduction band stresses the electronic configuration of the photoconductive compound, thereby reducing its band-gap and thus increasing its ability to release electrons to its conduction band. Phosphors almost necessarily imply the presence of such activating substances. The effect of such impurities may be such a to confer photoconductivity upon a compound which intrinsically is non-photoconductive. On the other hand, excessive impurity content can interfere with a compound acting as a photoconductor, as above described.

The photoconductors of this invention may be sensitized to visible and other wavelengths of light by foreign ion doping, addition of fluorescent materials, and/ or by means or sensitizing dyes. Bleachable dyes useful for sensitizing the photoconductors of this invention include, for example, the cyanine dyes, the dicarbocyanine dyes, the carbocyanine dyes, and the hemicyanine dyes. Additional dyes which are useful for sensitizing the photosensitive medium of this invention are the cyanine dyes described on pps. 371-429 in The Theory of Photographic Process, by C. E. Kenneth Mees published by McMillan Company in 1952. Other useful dyes include those known to the art as triphenylmethane dyes such as crystal violet and basic (Fuchsin, diphenylmethane dyes such as Auroamine O, and Xanthene dyes such as Rhodamine B.

Binders suitable for use with the copy media in this invention are those which can be hardened by application of a tanning agent. Included are water-permeable organic colloids, such as gelatin, albumin, casein, agar-agar, etc. Other binders which could be used with this invention are described in more detail in McDonald, U.S. Pat. 3,055,758

at column 2, line 60, through column 3, line 8, the teachings of which are hereby incorporated by reference.

The copy media are exposed to an image pattern of activating radiation which forms a latent image in the copy media by reversibly activating the photoconductor at the image-struck area. Exposure can be by any of the well-known techniques, as for example by exposing through a black and white or color transparency whereby a negative photographic relief image of the transparency will be produced by the process of this invention. By the term activating radiation is meant any activating electromagnetic radiation including but not being limited to actinic light, X-rays, or gamma rays or any other form of activating irradiation such as an electron beam.

Prior to exposure, the photoconductor should be deactivated by storing the copy medium in the dark from 1 to 24 hours prior to use, or by other known techniques such as heating, contacting with hydrogen peroxide or otherwise.

The latent image formed upon exposure is contacted with image-forming materials to form a germ image which may or may not be visible. These image-forming materials are solutions containing reducible ions which have at least the oxidizing power of the cupric ion. Included are such metal ions as Ag+ Hg+ Pb+ An, N, Pt+ Ni Sn+ Pb, Gu and Cu. Any suitable solvent may be utilized.

The concentration of reducible ions in the image-forming solutions can be adjusted so that sufiicient reducible ions are present to form a germ image suitable for tanning without forming substantial amounts of insoluble hydroxides which could be formed under the alkaline conditions required for tanning. As an example, if silver nitrate is used to form a germ image upon photoconductive materials of the type described in Gold, US. Pat. 3,380,823, the concentration of free silver ions present in the solution can be made much lower than that customarily used. This can be accomplished by using more dilute silver nitrate solutions such as those having silver nitrate concentrations of from about 1 molar to about 0.0001 molar and preferably those with silver nitrate concentrations of from about 0.1 molar to about 0.001 molar. Altemately, higher concentration silver nitrate solutions can be used with complexing agents such as ammonium hydroxide, ethylenediaminetetraacetic acid, citric acid or other silver complexing agents known to those skilled in the art. Acidic complexing agents should not be used in quantities which would cause the pH of the combined image-forming solution and tanning developer to be in the acidic range, however.

After the copy media have been exposed and contacted with image-forming materials, the hardenable binder is tanned by contacting it with a tanning developer. Some examples of suitable tanning agents include hydroquinone, chlorohydroquinone, catechol, 4-phenylcatechol, pyrogallol, p-methyl-aminophenol sulfate, etc. Other tanning developers suitable for use with this invention are described in the following references, the teachings of which are hereby incorporated by reference: Glafkides, Photographic Chemistry, vol. 1, Fountain Press, London (1960), pp. 664 et seq.; Pontius, R.B., PSA Journal, B (September 1951); and Tull, A.G., The Journal of Photographic Science, vol. II (1963 at pp. 1 et seq. Dihydroxybenzene tanning developers are preferred because of the excellent results obtained with such and an especially preferred tanning developer is hydroquinone.

In some instances, it may be desirable to combine the image-forming solution with the tanning developer in one solution. Normally, however, the copy medium is first contacted with the image-forming solution to form a germ image and subsequently a tanning developer is applied to harden the gelatin at the image-struck area. Also, one or both may be incorporated in the hardenable binder prior to exposure.

The pH of the tanning developers is maintained in the alkaline range. Preferably, the pH is maintained above about 9 for the best tanning development.

The remaining portions of the unhardened binder are removed by mechanical, chemical or other means to leave a photographic relief image on the copy medium. For example, if the exposure was through a transparency, a negative relief will have been reproduced on the copy medium which will have raised or hardened portions of the binder in the light-struck area.

The relief images produced by the process of this invention have many uses. For example, they can be used as transparencies whereupon a negative image of the original may be projected upon a screen. Also, the reilef images can be used as diazo masters to prepare standard diazo copies. In addition, the relief image can be used as a dye transfer matrix suitable for imbibing and transferring dyes. Moreover, some hardenable binders such as the gelatin are normally hydrophilic, but become olcophilic when sufficiently hardened or tanned, and relief images formed with such binders can be used as lithographic plates.

(Full-color dye transfer positive prints can be prepared from negative color transparencies using the photographic relief irnagies produced by the process of this invention by the following technique. A series of relief images is formed by exposing through red, green and blue filters so that one of the relief images will have raised portions corresponding to light which passes a red filter, another will have raised portions corresponding to light which passes a green filter, and a third will have raised portions corresponding to light which passes a blue filter. These relief images can be separately immersed in red, green and blue dyes and used to produce composite dye transfer prints. Such a process is further described in Sprague, UJS. Ser. No. 713,212.

The following examples further illustrate the invention. All parts and percentages are by weight unless otherwise specified.

EXAMPLE I A copy medium having a gelatin binder layer 0.14 mil thick joined to a triacetate film substrate and subbing layer having a combined thickness of 5.59 mils was used. The gelatin binder contained titanium dioxide with an average particle size of about 250 millimicrons and the pigment/binder ratio was approximately 1/ l.

The copy medium was exposed imagewise for one second using a Block Raymaster fluorescent light source, Model No. TF6, with a six watt bulb about three inches from the medium.

After 5 seconds, the exposed copy medium was immersed in a 0.1 molar aqueous solution of silver nitrate for a period of seconds, after which the copy medium was drained for 10 seconds and immersed in water at room temperature 10 seconds.

Tanning development was accomplished by immersing the copy medium in the following developer for seconds:

Grams Hydroquinone 0.5 Sodium hydroxide 2.0

Water to one liter.

The developer had a hydroquinone concentration of 4.55 X 10- molar and a pH of 12.7.

After tanning, the copy medium was washed with water at a temperature of 50 C. to remove the untanned gelatin binder. Water at 50 C. will dissolve and remove untanned gelatin within a few seconds. The hardness or permanence of the relief image formed was checked by measuring the image and background optical densities after various wash times.

The results were as follows:

50C. water 0D 0D image background wash time A 0D 10 seconds 0. 47 0. 04 0. 43 5 minutes. 0. 41 0. 04 0. 37

The slight decrease in A OD after a five-minute wash indicates that the relief image formed had good hardness and permanence.

EXAMPLE II The procedure of Example I was followed except that the hydroquinone concentration was lowered to 1.07 x 10- molar and the tanning time was varied to obtain the following results after washing for ten seconds:

Tanning time OD OD (sec.) image background A 0D EXAMPLE III The procedure of Example I was followed except that chlorohydroquinone and catechol were substituted for the hydroquinone at a concentration of 1.4 10- molar each. Relief images were formed which had good hardness and permanence.

EXAMPLE IV TABLE I.DYE TRANSFER Immersion time Transfer 0D OD 111 dye (see) time (see) image background What is claimed is:

1. A tanning process for forming a photographic relief image from a copy medium which has a base with a photoconductor dispersed in a hardenable binder thereon, said photoconductor being of the type which becomes reversibly activated upon exposure to activating radiation to cause the reduction of reducible ions applied thereto, comprising:

(a) exposing said copy medium to an image pattern of activating radiation which reversibly activates the photoconductor at the image-struck area;

(b) contacting the image-struck area with image-forming materials containing reducible metallic ions, which have at least the oxidizing power of the cupric ion, to thereby form a germ image, the concentration of free, reducible, metallic ions being suflicient to form a germ image suitable for tanning but below that concentration required to form substantial amounts of insoluble hydroxides at the tanning pH;

(c) contacting at least the image-struck area of said copy medium with a tanning developer to harden said binder at the germ image; and,

(d) removing the remaining portions of unhardened binder from said copy medium.

2. A process of claim 1 wherein said reversibly activatable photoconductor comprises a metal oxide selected ffiom titanium dioxide, zinc oxide or combinations of t ese.

3. A process of claim 1 wherein said hardenable binder comprises gelatin.

4. A process of claim 3 wherein said reducible metallic ions comprise silver ions.

5. A process of claim 4 wherein said image-forming materials comprise an aqueous solution of silver nitrate whose concentration is between 0.1 molar and 0.001 molar.

6. A process of claim 4 wherein said image-forming materials comprise an aqueous solution of silver nitrate which also contains a silver complexing agent.

7. A process of claim 5 wherein said tanning developer comprises hydroquinone.

8. A process of claim 6 wherein said tanning developer comprises hydroquinone.

9. A tanning process for forming a photographic relief image from a copy medium which has a base with a photoconductor dispersed in a hardenable binder thereon, said photoconductor being of the type that becomes reversibly activated upon exposure to activating radiation to cause the reduction of reducible ions applied thereto, comprismg:

(a) exposing said copy medium to an image pattern of activating radiation to reversibly activate said photoconductor at the image-struck area;

(b) contacting said image-struck area with a combined solution containing both image-forming, reducible, metallic ions, having at least the oxidizing power of the cupric ion, and a tanning developer, to simultaneously form a germ image and harden said binder at the image-struck area, the concentration of free, reducible, metallic ions in said combined solution being sufiicient to form a germ image suitable for tanning but below that concentration required to form substantial amounts of insoluble hydroxides at the tanning pH; and,

(c) removing the remaining portions of unhardened binder from said copy medium.

10. A process of claim 9 wherein said reversibly activatable photoconductor comprises a metal oxide selected from titanium dioxide, zinc oxide and combinations of these.

11. A process of claim 10 wherein said hardenable binder comprises gelatin.

12. A process of claim 11 wherein said reducible metallic ions comprise silver ions.

13. A process of claim 12 wherein said combined solution comprises an aqueous silver nitrate solution and a tanning developer.

14. A process of claim 13 wherein said combined solution has a silver nitrate concentration of from about 0.1 molar to about 0.001 molar.

15. A process of claim 13 wherein said combined solution also contains a complexing agent for silver ions.

16. A process of claim 14 wherein said tanning developer comprises hydroquinone.

17. A process of claim 15 wherein said tanning developer comprises hydroquinone.

References Cited UNITED STATES PATENTS 3,053,658 9/1962 Spencer 9633 3,414,410 12/1968 Bartlett et al 9627 2,257,105 9/ 1941 Champion 96-35 2,371,773 3/1945 Neumann 9635 X 3,138,458 6/1964 Kimble et al. 96-1.3 3,293,035 12/1966 Van Hoof et al. 9635 3,382,068 5/1968 Gold 96--l.5 X 3,409,429 11/1968 Ekman et al. 9627 FOREIGN PATENTS 1,043,250 9/1966 Great Britain 9627 CHARLES E. VAN HORN, Primary Examiner US. Cl. X.R. 9648 PD, 111 

